Spatiotemporally-resolved differential dynamic microscopy reveals strain-induced deformations in heterogeneous soft materials

Abstract

Understanding the microscopic behavior of materials in response to mechanical perturbations is a central challenge in soft matter, yet existing methods, which typically rely on microscopy-based imaging, often struggle with spatial heterogeneity and low signal-to-noise that are characteristic of many soft and biological materials. Additionally, many existing rheo-optical methods focus on the response to macro-scale forces rather than localized perturbations despite the importance of local stresses in biological and industrial applications of soft materials. Here, we present STroboscopically Averaged, time-Resolved Differential Dynamic Microscopy (starDDM) to investigate a sample's deformation across time and distance due to a cyclically applied local force. As proof-of-principle, we use this model-independent and tracer-free approach to map the motions of active cytoskeleton composites imposed by the movement of an optically trapped probe particle embedded within the complex network. We reveal how motor-driven restructuring and varying strain rates tune the magnitude and propagation of the response, which we further show decays with distance from the perturbation. This analytical platform fills a critical gap in micro-rheology, offering a versatile tool for probing the non-equilibrium dynamics of soft materials ranging from biopolymer networks to 3D-printed materials and soft robotics.